The field of the present invention relates to filters for polyphase rectifiers, and, more particularly, to a filter for permitting the reactive load currents to flow between the output terminals of the rectifier and a reference voltage or neutral terminal of the rectifier.
Polyphase rectifier systems, such as a three-phase diode-bridge rectifier, for example, are required in certain applications to provide dc power to a reactive load, while permitting reactive load current to flow between the output terminals of the rectifier and a reference voltage or neutral terminal of the rectifier. It is known to connect only capacitors between the output terminals and the reference voltage terminal of the rectifier for providing the current conduction paths for the reactive load currents. There are a number of problems associated with the use of such filter capacitors, including the requirement that the capacitors be high-power types, in that they must supply power to the load for a predominant portion of the period of the ac voltage source supplying power to the rectifier, whereby the capacitors are costly and physically large in size. In addition, the capacitors are supplied current for recharging only at times that the amplitude of the polyphase supply voltage to the rectifier exceeds by one diode drop the level of voltage across the capacitors, which usually occurs near the peak of the waveforms of the polyphase supply voltage, creating substantially high-magnitudes of peak current demand from the rectifier during the times of recharging the capacitor. Accordingly, the rectifier must be designed to supply the relatively high-magnitude of recharge current during the short intervals of recharging the capacitors over the period of the polyphase input voltage. Also, during a substantially predominant portion of the period of the polyphase supply voltages, the capacitors supply power to the load, not the rectifier. As a result, the polyphase voltage source or supply is poorly utilized.
The present inventor recognized that in many applications, polyphase rectifiers do not require filter capacitors for providing smoothing of the ac ripple super-imposed upon the dc output voltages of the rectifier, in that polyphase rectifiers have an inherently low ripple content in their output voltage. He discovered that the problems in the prior art could be overcome by including the parallel combination of a diode and resistor between the output terminals of the rectifier and their respective filter capacitors, with the diodes being polarized for permitting charging of their respective capacitors to the level and polarity of the voltage available at their respective output terminals. The value of the resistors are made large enough for substantially preventing the discharging of the capacitors over each cycle of the polyphase ac supply voltage, thereby blocking the flow of ripple current through the capacitors, and ensuring that the rectifier supplies substantially all of the power requirements of the reactive load over each cycle of the polyphase supply voltage, while permitting reactive load currents to flow bidirectionally between the output and reference terminals of the rectifier via the current conduction paths provided by the diodes, resistors, and capacitors.